# std::atomic_is_lock_free, ATOMIC_xxx_LOCK_FREE

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< cpp‎ | atomic

Defined in header `<atomic>` |  |
template< class T > bool atomic_is_lock_free( const volatile std::atomic<T>* obj ) noexcept; | (1) | (since C++11)
template< class T > bool atomic_is_lock_free( const std::atomic<T>* obj ) noexcept; | (2) | (since C++11)
#define ATOMIC_BOOL_LOCK_FREE /* unspecified */ #define ATOMIC_CHAR_LOCK_FREE /* unspecified */ #define ATOMIC_CHAR16_T_LOCK_FREE /* unspecified */ #define ATOMIC_CHAR32_T_LOCK_FREE /* unspecified */ #define ATOMIC_WCHAR_T_LOCK_FREE /* unspecified */ #define ATOMIC_SHORT_LOCK_FREE /* unspecified */ #define ATOMIC_INT_LOCK_FREE /* unspecified */ #define ATOMIC_LONG_LOCK_FREE /* unspecified */ #define ATOMIC_LLONG_LOCK_FREE /* unspecified */ #define ATOMIC_POINTER_LOCK_FREE /* unspecified */ | (3) | (since C++11)
#define ATOMIC_CHAR8_T_LOCK_FREE /* unspecified */ | (4) | (since C++20)

1,2) Determines if the atomic object pointed to by obj is implemented lock-free, as if by calling obj->is_lock_free(). In any given program execution, the result of the lock-free query is the same for all atomic objects of the same type.
3,4) Expands to an integer constant expression with value

- ​0​ for the built-in atomic types that are never lock-free,
- 1 for the built-in atomic types that are sometimes lock-free,
- 2 for the built-in atomic types that are always lock-free.

### Parameters

obj | - | pointer to the atomic object to examine

### Return value

true if *obj is a lock-free atomic, false otherwise.

### Notes

All atomic types except for std::atomic_flag may be implemented using mutexes or other locking operations, rather than using the lock-free atomic CPU instructions. Atomic types are also allowed to be sometimes lock-free: for example, if only some subarchitectures support lock-free atomic access for a given type (such as the CMPXCHG16B instruction on x86-64), whether atomics are lock-free may not be known until runtime.

The C++ standard recommends (but does not require) that lock-free atomic operations are also address-free, that is, suitable for communication between processes using shared memory.

### Example

Run this code

    #include <atomic>
    #include <iostream>
    #include <utility>

    struct A { int a[100]; };
    struct B { int x, y; };

    int main()
    {
        std::atomic<A> a;
        std::atomic<B> b;
        std::cout << std::boolalpha
                  << "std::atomic<A> is lock free? "
                  << std::atomic_is_lock_free(&a) << '\n'
                  << "std::atomic<B> is lock free? "
                  << std::atomic_is_lock_free(&b) << '\n';
    }

Possible output:

    std::atomic<A> is lock free? false
    std::atomic<B> is lock free? true

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3249 | C++11 | `atomic_is_lock_free` was specified via pointers, which was ambiguous and might accept invalid pointer values | specified via atomic objects

### See also

is_lock_free | checks if the atomic object is lock-free (public member function of `std::atomic<T>`)
atomic_flag (C++11) | the lock-free boolean atomic type (class)
is_always_lock_free [static] (C++17) | indicates that the type is always lock-free (public static member constant of `std::atomic<T>`)

std::atomic_is_lock_free(std::shared_ptr) (deprecated in C++20)(removed in C++26) | specializes atomic operations for std::shared_ptr (function template)
C documentation for atomic_is_lock_free
C documentation for ATOMIC_*_LOCK_FREE

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